This invention relates to solution heat pump systems and to methods for utilizing waste heat and more particularly to waste heat powered solution heat pump applications to up-grade waste heat by temperature boosting for use in various industrial applications for producing steam.
Many solution heat pump apparatus and methods have been developed. One of the first proposed practical uses of an absorption heat pump was reported by D. A. Williams and J. B. Tredemann at the Intersociety Energy Conversion Engineering Conference, 9th Proceedings, August, 1974 in a paper entitled Heat Pump Powered by Natural Thermal Gradients.
Additional work has been reported in various recent patents including the following:
U.S. Pat. No. 4,333,515, issued June 8, 1982, inventor William H. Wilkinson et al entitled Process and System for Boosting The Temperature of Sensible Waste Heat Sources;
U.S Pat. No. 4,338,268, issued July 6, 1982, inventor William H. Wilkinson et al, entitled Open Cycle Thermal Boosting System;
U.S. Pat. No. 4,402,795, issued Sept. 6, 1983, inventor Donald C. Erickson, entitled Reverse Absorption Heat Pump Augmented Distillation Process.
The foregoing patents and the references cited therein represent the current state of the temperature boosting art using solution heat pump technology and are incorporated herein by reference in their entirety.
In general, waste heat from industrial or other sources can be boosted to higher temperature levels by combining at least one relatively high pressure Rankine vapor generation cycle with at least one solution heat pump cycle. In a typical system, waste heat is utilized to boil off a fluid termed a refrigerant in the Rankine cycle evaporator to provide a source of vapor to an absorber in the solution heat pump. In the absorber, the refrigerant vapor is contacted with a binary working solution containing absorbent and refrigerant. As the refrigerant vapor is absorbed into the binary absorbent solution, its latent heat of condensation and heat of solution are given off to a heat exchanger at a temperature higher than the temperature of the waste heat source. The working solution is then throttled to reduce the pressure and introduced into a relatively low pressure desorber where a portion of the refrigerant is desorbed as vapor from the binary solution by the addition of more waste heat through a heat exchanger. The desorbed refrigerant vapor is then condensed by contact with a colder heat exchanger at a temperature less than the temperature of the vapor, and the condensed refrigerant is then pumped to the evaporator for reuse. The concentrated working solution is recycled from the desorber to the absorber through a heat exchanger where sensible heat is exchanged with the dilute working solution being conveyed from the absorber to the desorber.
Waste heat sources which have been used to power solution or absorption heat pumps, as described, can be obtained from either sensible heat, latent heat or both. Utilization of a sensible waste heat source has been maximized by extracting successive portions of heat for use first in the Rankine cycle evaporator section and then in the heat pump cycle desorber section of solution or absorption heat pump. Multiple cycle systems can also be employed to boost the temperature of a portion of the waste heat to even higher levels.
Many industries must dispose of large amounts of heat produced for or resulting from chemical processing and the like, which generally cannot be recovered using conventional heat exchange equipment because that heat is at too low a temperature for further use. Sources of this wasted heat include heat losses from boilers, drying equipment, chemical reactors, and fractionation equipment; low pressure steam which would otherwise be vented or condensed using air or cooling water and the like; and other low quality heat derived from a wide variety heat exchange equipment. In many cases, substantial amounts of increasingly expensive fuel must be burned only to result in much of the heat produced being lost in a low grade form of waste heat. If a portion of this waste heat could be upgraded for further use, energy would be conserved and fuel cost savings realized.
Several types of heat pumps can be used to increase the temperature of waste heat such as can be obtained from low pressure steam to a useable level. An absorption cycle heat pump process such as previously described may be utilized for this purpose.
A modification of an absorption cycle heat pump is described in U.S. Pat. No. 4,167,101 as a means to elevate the temperature of a waste heat source. In that substantially isobaric process, a vapor is absorbed into a liquid phase solvent in an absorption zone which subsequently releases its heat of solution to an external heat receiving medium. The solution is then taken to a stripping zone where a stripping gas desorbs the vapor from solution.
The resulting gaseous mixture is then fractionated by partial liquefaction and phase separation. The stripping gas is then recycled to the stripping zone while the liquid fraction is vaporized and then recycled to the absorber where the process is repeated.
The proposed use of a waste heat powered absorption cycle heat pump in a wide variety of industrial applications is most useful if the output of such a device is in the form of low to medium pressure process steam, since such steam is universally useful and easily conveyed within most processing plants without additional equipment. A temperature booster, to be economically useful in a variety of industrial applications where process steam is desired, should be able to exhibit a thermal efficiency of at least 40% per stage of temperature boost. In order to produce usable medium pressure process steam (i.e. up to 250 psig and 406.degree. F.), a temperature booster should also be capable of providing a maximum temperature boost up to nine-tenths of the temperature difference between the waste heat and the low temperature heat sink used for waste heat rejection. For example, if waste heat with an average temperature of 220.degree. F. and cooling water at 90.degree. F. were available, the maximum boosted output would be 340.degree. F. This relationship must generally hold true in order to produce an economically useful device for most steam producing industrial medium pressure steam producing applications. Generally, the waste heat that drives a temperature booster machine is energy that is not hot enough to be useful with conventional technology. It is therefore an objective of the present invention to provide an absorption cycle heat pump booster system in a method which is capable of economically upgrading waste heat to useful levels and in particular, to provide a system and method for producing high quality, low to medium pressure process steam for a wide variety of applications using relatively low quality waste heat as the waste heat source.